Band-pass filter of wide frequency range



Oct. 22, 1940. G ER 2,219,027 4 BAND-PASS FILTER OF WIDE FREQUENCY RANGE Filed Dec. 14, 1959 INVENTOR G'U/VT/IEP WEBER ATTORNEY Patented Oct. 22, 1940 PATENT OFFICE 2,219,027 BAND-PASS FILTER F WIDE FREQUENCY RANGE j Giinther Weber, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic in. b. H., Berlin, Germany, a corporation of Germany Application December 14, 1939, Serial No. 309,151 In Germany November 29, 1938 Claims.

Wide band intermediate frequency amplifiers for a frequency band of 200 to 600 kilocycles (1500 to 500 m.) were hitherto constructed in the following manner: Highly damped band filters consisting of coupled oscillatory circuits are employed, or individual relatively detuned circuits which are likewise damped and arranged in successive tube stages. Where a high amplification is to be attained with these methods, it is necessary to employ very low circuit capacities, i. e. circuit capacities and tube capacities are employed exclusively. This entails highly unstable conditions since very slight stray actions of the capacities are sufiicient already to deform appreciably the resonance curve. In the wide band intermediate frequency amplifiers with band filter a further disadvantage resides in the necessary high damping which entails a diminishing of the amplification. In the amplifier with relatively detuned circuits another disadvantage lies in the fact that each tube amplifies effectively but a part of the entire frequency band. In summarizing it can thus be said that both methods have the drawback of difiicult matching, eventually unstable conditions and a large requirement in tubes.

The invention proposes an arrangement which eliminates to a great extent these drawbacks.

In accordance with the invention the bandpass or admission filter is provided in that there is placed between the input terminals a series resonance circuit tuned approximately to the one limit frequency lying within the admission range, and that furthermore there is connected parallel to the one wattless resistance of the series resonance circuit a further series resonance circuit approximately tuned to the other limit frequency lying within the admission range and with the same sequence of the inductive and capacitive wattless resistances, and that the output potential is derived at the ends of the series connection of the two identical wattless resistances of the two circuits.

Figure 1 shows an example of construction according to the invention, and Figure 2 is a characteristic curve of the filter network disclosed in Figure 1.

The two series resonance circuits are designated L1, 01 and L2, C2. The capacity Ci of the series resonance circuit L1, Ci forms, to be exact, at the same time a part of the oscillatory circuit capacity of the other series resonance circuit but the capacity C1 varies but slightly the natural resonance of the circuit L2, C2, since the capacity 01 is high as compared with the capacity C2 (for instance 01:60 mmf. against 02:4 mmf. for L1=10 mh. and L2=6 mh.). The capacity C: is in fact higher than the capacity C1 in order that the admission curve has the same height at the two ends of the range. The difference between the capacities is the greater the wider the band width.

The resistance R having a value, for instance, of 100 megohms serves for applying the direct plate potential and for setting the most favorable amplification and at the same time it brings about a certain damping. The potential is derived at the condensers Cl and G2.

The possibility also exists to interchange the coils and condensers at all places in which case the filter curve according to Figure 2 in which the output potentials U are plotted on the vertical and the frequency f is plotted on the horizontal, has the reverse pattern.

An advantage of this filter resides in that at the same time a wide band width and a high amplification are attained. Furthermore, the filter is to a wide degree independent of the circuit capacities and tube capacities as was found from tests. Hence, it is possible to match this filter proper outside the amplifier.

The filter requires but a slight damping for instance by means of the resistance r since several maximi caused by resonances appear within the admission range. The filter curve hence has no deep impression contrary to a band filter composed of coupled oscillatory circuits and owing to the said resonance places the central range of the filter lies higher. This condition assures the one advantage of the filter, namely the higher transmission ratio despite the wide band width.

What I claim is:

1. Admission filter for a very wide frequency range, more especially for intermediate frequency amplifier, characterized in that there is placed between the input terminals, a series resonant circuit tuned approximately to the one limit frequency lying within the admission range, and that furthermore there is connected parallel to the one wattless resonance of the series resistance circuit a further series resonant circuit'approximately tuned to the other limit frequency lying within the admission range and with the same sequence of the inductive and capacitive wattless resistances, and that the output potential is derived at the ends of the series connection of the two identical wattless resistances of the two circuits.

2. Admission filter according to claim 1, characterized in that the capacity of the first series resonant circuit is substantially higher than the capacity of the other series resonant circuit.

3. A band-pass filter adapted to transmit a comparatively wide range of frequencies with substantially uniform response, comprising a pair of input terminals, 3. series circuit including an inductance and a capacity resonant to approximately one limiting frequency of the range to be passed connected between said terminals, a second series circuit including an inductance and a capacity resonant to approximately the other limiting frequency connected across the inductance of the first circuit, and a pair of output tercuit is substantially higher than the capacity of 10 the second series resonant circuit.

GtTN'rHER WEBER. 

